opkg: gracefully handle missing $PATH, fix build warnings

opkg: gracefully handle missing $PATH, fix build warnings

Update to latest Git in order to import the following fixes:

cfe46c7 opkg_active_list_test: mark functions static, swap strcmp() with memcmp()
0b60d6a libopkg_test: mark functions static, remove unused opkg_test()
28b5e15 pkg_parse: fix bad message level for opkg_msg() call
d9e5814 pkg_parse: remove unused variable in pkg_parse_line()
1654bcc pkg_parse: remove const requirement from pkg_parse_line()
53e199e pkg_hash: remove unused variable in pkg_hash_load_feeds()
da8d599 pkg_depends: add missing parse_replacelist() prototype
f0c0a80 opkg_utils: add missing header
e450488 libopkg: remove unused progress parameters from opkg_download()
f4a55d1 libopkg: remove unused progress callback
7cbc466 libopkg: remove unused variable from opkg_install_package()
6fbf8a9 libbb: remove unused variables
7975c73 libbb: change gzip_read() buffer argument to void *
6e28563 libbb: fix gzip_thread() warnings
1ff2475 libopkg/opkg_cmd.c: more robust PATH handling
5dc3e33 libopkg: specify "/bin/sh" instead of "sh"

Signed-off-by: Jo-Philipp Wich <jo@mein.io>